Whenever the fuel tank or the fuel line of an engine is exposed to cold weather, the fuel or certain impurities or additives contained in the fuel may gel or "wax", i.e., they become excessively thick. For example, the diesel fuel in the fuel tanks or fuel lines of diesel trucks may wax if the truck is parked or driven in extremely cold weather. Since waxing can interfere with the proper flow of fuel to the engine, it frequently results in poor engine performance. Severe waxing may even block the flow of fuel entirely, killing the engine.
Waxing may be eliminated by heating the fuel. However, while many devices for heating fuel to eliminate waxing have been proposed, they nonetheless have several undesirable characteristics. For example, one device utilizes a tube which loops through the fuel tank. Hot exhaust gas or coolant that has been heated by the engine is channeled through the tube, heating the fuel. However, it frequently requires a considerable period of time for the coolant or exhaust gas to become hot and for the hot coolant or exhaust gas to sufficiently heat the fuel in the tank to eliminate any waxing, especially for large fuel tanks. Consequently, the engine may nonetheless perform poorly for a long period of time.
Further, while much of the fuel is eventually heated by such a device, localized cold spots may still develop. For example, in extremely cold weather, the fuel may continue to wax along the bottom and sides of large metal fuel tanks. Since the inlet of the fuel line is typically located near the bottom of the fuel tank, such localized waxing along the bottom of the tank may interfere with the proper flow of fuel through the fuel line.